【作者】 邓书康；

【导师】 唐新峰；

【作者基本信息】 武汉理工大学 ， 新能源材料， 2008， 博士

【摘要】 近年来，笼式化合物由于其优越的热电性能及在热电发电领域的潜在应用前景引起了研究者的广泛关注，Ⅰ-型Ge基笼合物为其重要代表性化合物之一。电子态密度分析表明，在化学式为Ⅱ₈Ⅲ₁₆Ⅳ₃₀的Ⅰ-型笼合物中，每个填充原子提供两个电子（作为施主）全部填充到框架上（作为受主），因而电价是平衡的，其应表现为本征半导体特性。然而研究表明大部分Ⅰ-型笼合物为金属特性和n传导。虽然目前有一些关于p型传导的Ⅰ-型笼合物的报道，但与具有较高ZT值的n型传导Ⅰ-型笼合物相比其热电性能较低，从而限制其实际应用。因此研究和开发高性能的p型Ge基Ⅰ-型笼合物非常必要和迫切。本论文分别以Zn、Cd、Al和Mg为掺杂元素，对具有n型传导特性的Ⅰ-型Ba₈Ga₁₆Ge₃₀笼合物的框架原子Ge位进行掺杂，制备了p型传导的Ⅰ-型笼合物，研究不同掺杂元素及掺杂量对其结构及其热电传输特性的影响规律，得到的主要结论如下：用熔融法结合放电等离子烧结（SPS）方法合成了不同Zn掺杂量的p型Ge基Ba₈Ga₁₆Zn_xGe_30-x（x=2．4，2．6，2．8，3．0，3．2）笼合物。由于Zn的原子质量和共价半径与Ge的相比差别不大，因此Zn的掺入对化合物的结构及原子位移参数（ADP）无明显影响。随Zn取代量x的增加，化合物载流子浓度N_p增加，载流子迁移率μ_H、电导率σ和热导率k逐渐降低。在低温区域（T<200 K），p型Ba₈Ga₁₆Zn_xGe_30-x化合物载流子散射主要以杂质电离化散射机制为主，随温度的升高，散射机制逐渐过渡为以声学波散射为主。在所有样品中，Ba₈Ga₁₆Zn₃Ge₂₇化合物ZT值最大，在790 K时达0．38。研究微调Ga含量对Ba₈Ga_16+xZn₃Ge_27-x热电性能的影响。随Ga含量的增加，化合物载流子浓度和电导率逐渐增加，Seebeck系数和载流子迁移率逐渐降低。化合物的热导率随Ga含量的增加而增加。在所有样品中，p型Ba₈Ga_16．2Zn₃Ge_26．8化合物的ZT值最大，在720 K时达0．43，与p型Ba₈Ga₁₆Zn₃Ge₂₇相比ZT值提高了13％。以Cd为掺杂元素制备了p型传导的Ge基Ba₈Ga₁₆Cd_xGe_30-x（x=0．9，0．95，1．0，1．05）Ⅰ-型笼合物。研究表明，与Zn掺杂样品相比，Cd的ADP明显高于其它框架原子的值。随Cd掺杂量的增加化合物的电导率逐渐增加，Seebeck系数逐渐降低，且达到最大值的温度逐渐向低温方向移动。在所有样品中当Cd的掺杂量x=1．0时对应样品有最大的功率因子，在550 K时达0．35×10^-3Wm^-1K^-2。在室温附近Ba₈Ga₁₆Cd_1．0Ge_29．0化合物的晶格热导率与Ba₈Ga₁₆Zn₃Ge₂₇化合物相比降低6％左右。Ba₈Ga₁₆Cd_1．0Ge_29．0化合物的最大ZT值在600 K时达0．173。以过量的Ⅲ族元素Al作为掺杂元素合成了p型Ba₈Ga₁₆Al_xGe_30-x（x=1．0，2．0，3．0，4．0，5．0）Ge基笼合物，研究表明，Al在框架上优先占据晶体学6c位置，Al的掺入对框架原子和填充原子的ADP无明显影响，Al的ADP与其余框架原子的ADP相当。当x≤4．0时，随Al取代量的增加，化合物电导率逐渐增加，Seebeck系数逐渐降低，当Al的名义含量x=5．0时，由于第二相BaAl₄的电导率较高，使对应样品的电导率大幅度增加。由于Al原子与框架原子之间较大的质量波动，使Al掺杂化合物的晶格热导率较低，在所有样品中，Ba₈Ga₁₆Al_3．0Ge_27．0化合物的晶格热导率最低，在300 K处达0．96 Wm^-1K^-1。当Al的名义含量x=3．0时对应样品Ba₈Ga₁₆Al_3．0Ge_27．0化合物在720 K处得到最大ZT值达0．61。微调Ga后Ba₈Ga_xAl₃Ge_43-x化合物的综合电性能得到一定程度的优化，当Ga的名义含量x=16．1时，对应化合物的Z殖在720 K时达0．63。通过Mg对框架原子Ge进行取代制备了p型传导的Ba₈Ga₁₆Mg_xGe_30-x（x=1．0，2．0，3．0，4．0）Ⅰ-型笼合物。分析表明当Mg的名义含量≤3．0时，所制备的化合物为单一的具有空间群为pm（?）3n的简单立方结构的Ⅰ-型笼合物，而当Mg的名义含量为4．0时，所制备的样品由Mg含量较高的和少量的Mg含量较低的两种Ⅰ-型笼合物组成。随着Mg含量x的增加，化合物的电导率逐渐降低，Seebeck系数逐渐增加。在总热导率中，晶格热导率占主导且晶格热导率随Mg含量的增加而逐渐降低。在室温附近和750 K附近，Ba₈Ga₁₆Mg_4．0Ge_26．0笼合物的晶格热导率分别为1．17 Wm^-1K^-1和0．81 Wm^-1K^-1，与相同温度下Al掺杂p型笼合物Ba₈Ga₁₆Al_3．0Ge_27．0的晶格热导率相当，比Zn掺杂p型笼合物的低。p型Ba₈Ga₁₆Mg_3．0Ge_27．0笼合物的无量纲性能指数ZT在740 K处达到最大值0．32。更多还原

【Abstract】 In recent years, clathrates with cage structures have gained renewed attention due to their promising thermoelectric properties and potential application in thermoelectric generator and type-Ⅰgermanium clathrate is one of them. Electron density analysis has shown that inⅡ₈Ⅲ₁₆Ⅳ₃₀ type-Ⅰclathrates, each guest atom （as donor） donates two electrons to the framework （acceptor） and the total electrons donated by the guest atom are accepted completely by the framework, and hence it should behave intrinsic semiconduction characteristic. However, most reports in literature show samples with metal-like and n-type properties. There are some p-type samples have been reported. However, the thermoelectric property of p-type clathrates is very low compared with that of n-type Ge-based clathrate with high ZT value, so it is very urgent and necessary to develop and investigate p-type clathrates with high performance.In this thesis, using Zn, Cd, Al and Mg to replace the framework atom Ge of type-ⅠBa₈Ga₁₆Ge₃₀ clathrate which with n type conduction, the type-Ⅰclathrates with p type conduction characteristics were synthesized successfully, and the main obtained results as follows.Zn-doping Ba₈Ga₁₆Zn_xGe_3o-x（x=2.4, 2.6, 2.8, 3.0, 3.2） p type clathrates with different Zn contents were synthesized by combing solid-state reaction with spark plasma sintering （SPS） method. The effects of Zn doping on the structure and on the atom displacement parameters （ADP） is not obviously due to difference of the atom mass and the covalence radius between the Zn and Ge is not obvious. The carrier concentration N_p increases while the electrical conductivity, and the carrier mobility and the thermal conductivity decrease with the increasing Zn content for Zn doped Ba₈Ga₁₆Zn_xGe_3o-x compounds. The carrier scattering mechanism is mainly ionized impurity mechanism in the low temperature range （7<200 K）, and gradually change to an acoustical mechanism with the rising temperature. The maximum ZT value of 0.38 is obtained at 790 K for Ba₈Ga₁₆Zn_3.0Ge_27.0 compound. The effects on the thermoelectric properties of Ba₈Ga_16+xZn_3.0Ge_27.0 were studied by slightly adjusting the Ga content, The carrier concentration, the electrical conductivity and the thermal conductivity increase, while the Seebeck coefficient and the carrier mobility decrease with the increasing of Ga content. The highest ZT value of 0.43 is obtained for the specimen with x=16.2 at 720 K, which is increased by 13% compared with that of Ba₈Ga₁₆Zn_3.0Ge_27.0 compound.P-type Ba₈Ga₁₆Cd-xGe_30-x （x=0.95, 1.0, 1.05, 1.10） type-Ⅰclathrates were synthesized using Cd as doping element. The ADP of the Cd atom is much larger than that of other framework atoms compared with that of Zn doped clathrate. The electrical conductivity increases while Seebeck coefficient decreases with the increasing Cd content, and the temperature which the Seebeck coefficient reaches its maximal value shift to the low temperature range with increasing Cd content. The maximum power factor value of 0.4×10^-3 Wm^-1K^-2 is obtained at 550 K for Ba₈Ga₁₆Cd_1.0Ge_29.0.The room temperature thermal conductivity for Ba₈Ga₁₆Cd_1.0Ge_29.0 is reduced by 6% compared with that of Ba₈Ga₁₆Zn₃Ge₂₇ compound. The largest ZT of 0.173 is obtained for Ba₈Ga₁₆Cd_1.0Ge_29.0 at 600 K.P type Ba₈Ga₁₆Al_xGe_30-x（x=1.0, 2.0, 3.0, 4.0, 5.0） type-Ⅰclathrates were synthesized by excess group-ⅢAl element. The results reveal that the Al atom reference for the 6c site. The Al substitutions do not affect the ADP of framework atoms （Ge/Ga） and filled atoms （Ba） compared with that of Ba₈Ga₁₆Ge₃₀; and the ADP of Al are nearly equivalent to that of other framework Ge/Ga atoms. The carrier concentration and electrical conductivity increase while Seebeck coefficient decreases with the increasing Al content for the specimens with x≤4.0. But for the sample with x=5.0, the electrical conductivity increase distinctly due to the higher electrical conductivity of BaAl₄.The lattice thermal conductivity of Al doping compounds are relatively lower due to the mass fluctuation between Al atoms and other atoms in the framework; it is as low as 0.96 Wm^-1K^-1 at 300 K for Ba₈Ga₁₆Al_3.0Ge_27.0 compound. The maximum ZT value of 0.61 is obtained at 760 K for Ba₈Ga₁₆Al_3.0Ge_27.0. The electrical properties of the Al doping p type Ba₈Ga_xAl₃Ge_43-xclathrate are enhanced by slightly adjusting the Ga content, and the ZT value of 0.63 is obtained for the specimen with x=16.2 at 810 K.Using Mg atom to substitute for the framework atom Ge of type-Ⅰclathrate Ba₈Ga₁₆Ge₃₀, p-type Ba₈Ga₁₆Mg_xGe_3o-x（x=1.0, 2.0, 3.0, 4.0） type-Ⅰclathrates were synthesized. The analysis results indicate that the specimens with≤3.0 are single type-Ⅰclathrates with cubic structure （space group pm （3|-） n）. But, for the specimen with x=4.0, two different type-Ⅰclathrates which with two different Mg content are detected: the main phase with higher Mg content, and the second phase with lower Mg content. The electrical conductivity decreases and Seebeck coefficient increases with the increasing of Mg content. The lattice thermoelectric is dominant part in the total thermoelectric conductivity, and it decrease with the increasing Mg content. The lattice thermal conductivity of Ba₈Ga₁₆Mg_3.0Ge_27.0 compound is 1.17 Wm^-1K^-1 and 0.81 Wm^-1K^-1 at the room temperature and 720 K respectively, which is correspond to that of Al doping p type Ba₈Ga₁₆Al_3.0Ge_27.0 at the same temperature and lower than that of Zn doping p-type clathrates. The dimensionless figure of merit ZT of 0.32 is obtained for the p type Ba₈Ga₁₆Mg_3.0Ge_27.0 clathrate.更多还原